I'm not an alarm expert but did self-install my G2-12 with some help from this forum. I reckon that if you've disconnected the phone line they won't be able to access any settings, but neither will you without the engineers code, which they won't give you. Have a read through the installation manual and see if you can reset the panel to factory settings so you can gain full control of it. Be aware that ADT seem to 'customise' some panels so yours might not actually be a G2 - my old ADT panel had stickers on it stating it was a Galaxy 8, and had user manuals for a Galaxy 8, but the motherboard wasn't from a Galaxy 8. New G2-12 panels are on ebay for about £30 if you need one.
Your comms fault can be disabled but you need engineer mode access. Also check that you have an external siren - some ADT systems designed to call their Alarm Receiving Centre (ARC) don't. And be aware that the built-in PSTN dialler is designed to act on a 'handshake' from the ARC, and while it should be fairly easy to set it to dial a number of your choice, it won't hang that call up unless it gets it's 'handshake', thus rendering that phone useless and stuck on an incoming call from your alarm.
Have a google at Selfmon, it's a service where you buy a £50 ethernet module then pay £1/month, which gives you full control of your panel via phone app etc, including text messages etc if the alarm is triggered. I've been a happy customer for about 4 years now, as have others on here.
Others on here have far more knowledge than me, but hopefully my info will give some food for thought.
